fastboot: add command to select the eMMC boot configuration

Add command oem bootbus which executes the command
``mmc bootbus <id> <arg>`` on the current fastboot mmc device
(<i> = CONFIG_FASTBOOT_FLASH_MMC_DEV) to set the eMMC boot
configuration on first update, with
<arg> =  boot_bus_width reset_boot_bus_width boot_mode

$> fastboot oem bootbus:<boot_bus_width> <reset_boot_bus_width> <boot_mode>

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
diff --git a/drivers/fastboot/Kconfig b/drivers/fastboot/Kconfig
index 1bcc8d4..a17e488 100644
--- a/drivers/fastboot/Kconfig
+++ b/drivers/fastboot/Kconfig
@@ -196,6 +196,13 @@
 	  Add support for the "oem partconf" command from a client. This set
 	  the mmc boot-partition for the selecting eMMC device.
 
+config FASTBOOT_CMD_OEM_BOOTBUS
+	bool "Enable the 'oem bootbus' command"
+	depends on FASTBOOT_FLASH_MMC && SUPPORT_EMMC_BOOT
+	help
+	  Add support for the "oem bootbus" command from a client. This set
+	  the mmc boot configuration for the selecting eMMC device.
+
 endif # FASTBOOT
 
 endmenu